Decayed wood is often not initially visible on the wood surface. For instance in the case of utility poles, decay often occurs in the portion of the pole below the ground. This makes it difficult to determine visually whether decay is present in the wood, particularly when decay generally occurs in the heartwood at the center of the pole and cannot be determined from the outside without drilling a hole into the wood. In present day examination of utility poles, it is normal to drill a hole downwards from above the ground line into the center of the wood so that a sample of the wood from the heart or core is extracted. In some cases decay is visible, however if the decay is incipient, then it is difficult to detect.
Vibration techniques have been suggested for determining decay in poles Shaw in U.S. Pat. No. 3,877,294 disclosed a vibration technique. Heath et al U.S. Pat. No. 3,531,983 related to Sonic testing. Miller et al in U.S. Pat. No. 3,521,483 and Shigo in U.S. Pat. No. 3,864,627 utilize electrical signals to detect the presence of decay.
There is also a need to determine if decay is present in standing trees, logs, and lumber being processed in sawmills, pulpmills or other manufacturing plants, as well as wood in use. Decay is often not visible from the exterior of trees, boards, or other wood products. Detection of decay in these situations would allow manufacturers to select the highest value processing option for individual pieces. In the case of wood in use, decay detection would allow engineers, or others responsible for the maintenance of structures, to determine when and if remedial steps are necessary. Some indication of the severity of decay or residual strength of the wood member would also be useful.
In copending application Ser. No. 444,878 filed Dec. 4, 1989, now U.S. Pat. No. 5,071,771 is disclosed a method of producing an ion mobility signature representing a wood sample and then comparing the signature with known signatures of wood species to determine the species of the sample. There is a need to determine not only the species but also to detect decay in a log or board at the same time that the species is determined. The time that is available for detecting decay at a sawmill is about one half to ten seconds per sample.